The use of ion selective electrodes (ISEs) to determine the presence and quantity of various analytes in biological samples has become a useful diagnostic technique. Indeed, ISEs have been used to detect analytes such as magnesium, sodium, potassium, calcium, and chloride, among others. Some of these ISEs are often housed within clinical diagnostic instruments for simultaneous analysis of a large number of analytes.
For several reasons, surfactants are often included in reagents used during the operation of ISEs. Various surfactants may be used for this purpose; however, the utility of the surfactant is highly dependent upon the ISEs membrane. For example, an unsuitable surfactant can result in a shift in electromotive force (EMF) bias that does not allow the electrode to measure a biologically relevant amount of an analyte. Further, for electrodes housed within a clinical diagnostic instrument, it is preferred that the surfactant used in the reagents for multiple ISEs is the same or similar. This reduces costs and the potential for contamination.
Issues associated with surfactant selection have led to difficulty in incorporating magnesium ISEs into clinical diagnostic instruments. Alkyl-N-methylglucamide-based non-ionic surfactants, such as N-methyl-noctanoyl-D-glucamine (sold under the tradename MEGA 8) or N-methyl-N-nonanoyl-D-glucamine (sold under the tradename MEGA 9), are recommended by IFCC guidelines used in reagents for magnesium ion selective electrodes. Although these surfactants are suitable for use with magnesium ion selective electrodes, they have significant drawbacks. First, the costs of these surfactants are extremely high. Further, these surfactants have a high critical micelle concentration (CMC), requiring a higher concentration of the surfactant in the reagent.
Because of the prohibitive cost of these surfactants, less expensive surfactants are desirable to use in these reagents. When used with magnesium ion selective electrodes, these less expensive surfactants make the electrode inoperable. As such, magnesium ion selective electrodes have only very rarely been included in clinical diagnostic instruments.
Accordingly, a surfactant that does not lead to an unacceptable EMF shift bias when used with a magnesium ISE, that is less expensive than traditional surfactants used with magnesium ISEs, and that may be used as a surfactant for other electrodes within a clinical diagnostic instrument would be highly desirable. The methods and kits of the present invention are directed toward these, as well as other, important ends.